(1) Field of the Invention
The present invention relates to a heavy duty pneumatic radial tire, and more particularly, the invention relates to a heavy duty pneumatic radial tire, for instance, a tire for use in the construction vehicles. Such tires are advantageously adapted for use in running on rough roads at a considerably high speed, wherein durability of belt is improved by effectively avoiding belt separation which is caused by shear strain at edges of the belt without deteriorating wear resistance and uneven wear resistance of such a tire.
(2) Description of the Prior Art
In such a radial tire, the tread portion of the tire is generally reinforced by superimposing a belt composed of plural rubberized cord layers upon the outer periphery of a radial structure carcass composed of one or more rubberized cord layers containing cords each arranged in a radial plane of the tire or in a plane deviating at a small angle therefrom.
In general, the belt is broadly classified into the following two kinds in terms of the structure:
(i) The cords are arranged at an angle of as small as about 20.degree.-25.degree. with respect to a plane containing the central circumference of the tire, that is, the equatorial plane of the tire, so as to restrain the enlargement of the outer diameter caused when the tire contacts the ground during running.
(ii) The cords of the nearest belt layer to the carcass and positioned on the innermost side are arranged at an angle of about 65.degree.-70.degree. with respect to the equatorial plane, and the cords of the remaining belt layers are arranged at an extremely small angle with respect to the equatorial plane, so that triangle link structure is formed between the carcass cords and the belt cords to give a high rigidity to the crown portion of the tire.
As to the item (i), since the shear strain between the cord layers at the edge portion of the belt becomes excessive, which is caused by the tread portion contacting the ground, separation problems at that position are likely to develop, while as to item (ii), although the occurrence of the shear strain when the internal pressure is applied is low due to its high rigidity in a direction along the tire equator, the belt is forcedly deformed while running on a rough road of an uneven surface on which rocks and/or stones are scattered by the deformation of the tread portion when it contacts the ground. Hence, the portions near the edges of the belt fixed in the triangle link structure exceed the permissible shear strain, wherein a separation problem is likely to occur.
Although there have been various examinations directed to a reduction in the strain at the belt edges through making proper the belt width, cord angle and cord number per unit length, and changing the quality of the cords for the belt and the carcass line to avoid the belt edge separation (hereinafter referred to briefly as "end separation") in the heavy duty pneumatic radial tire for use in the running road containing the above rough road as at least a part thereof, there is naturally a limit on the improvement by such a method, since the necessary rigidity of the belt and the strain of the belt edges are just in the reverse relation with each other, so that when the rigidity of the belt is lowered to decrease the strain, a demerit inevitably takes place with respect to the wear resistance and the uneven wear resistance of the tread.
Meanwhile, as to the tread patterns, the so-called lug type pattern is commonly used because the conventional tire of this type generally needs strong traction, but there has not been reported in the literature any studies in which attention was directed to examine the tread pattern as the governing factor of the strain behavior at the tread portion, particularly, the belt edge portions.